Electrocatalyst for water electrolysis

ABSTRACT

A cathode is provided for electrolysis of water wherein the cathode material comprises a multi-principal element, transition metal dichalcogenide material that has four or more chemical elements and that is a single phase, solid solution. The pristine cathode material does not contain platinum as a principal (major) component. However, a cathode comprising a transition metal dichalcogenide having platinum (Pt) nanosized islands or precipitates disposed thereon is also provided.

RELATED APPLICATION

This application claims benefit and priority of U.S. provisionalapplication Ser. No. 62/766,165 filed Oct. 4, 2018, the entiredisclosure of which is incorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Contract No.DE-AC02-07CH11358 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the electrochemical splitting of waterand, more particularly, to an electrocatalyst comprising certaintransition metal chalcogenide materials for use as the cathode for theelectrochemical hydrogen evolution reaction.

BACKGROUND OF THE INVENTION

Hydrogen (H₂) is a unique energy carrier in that it can be produced viadiverse pathways utilizing a variety of domestically availablefeedstock, including natural gas, biomass, and water (reference 1).

The electrochemical splitting of water (electrolysis) is among the mostversatile methods of hydrogen generation that is expected to play asignificant role in long-term, high-volume hydrogen gas production(reference 2).

The electrolysis of water is a combination of two space separatedelectrochemical processes—the hydrogen and oxygen evolution reactions(HER and OER, respectively). During HER, water is reduced to H₂ usingelectrons supplied by the negative electrode (cathode) of anelectrochemical cell, while oxygen (O₂) is produced by OER at itspositive counterpart (anode) (FIG. 1). Since HER and OER are separatedin space, the produced hydrogen and oxygen require only limited, if any,purification.

In an ideal electrochemical cell, operating at 25° C. under atmosphericpressure (760 mmHg or 101325 Pa, or 14.696 psi), oxidation of wateroccurs at the anode potential of E^(o) _(a)=1.23V and the hydrogenevolution takes place at the cathode at E^(o) _(c)=0 V (standardhydrogen electrode).

Anode (OER): H₂O(aq)→½O₂(gas)+2H⁺(aq)+2e ⁻

Cathode (HER): 2H⁺(aq)+2e ⁻→H₂(gas)

Thus, the voltage to be applied to an ideal electrolyzer cell is E^(o)_(cell)=E^(o) _(c)−E^(o) _(a)=−1.23V. However, in practice, waterelectrolyzers operate at higher cell voltages due to energy losses bynon-ideal electrochemical processes. The fraction of the electricalpotential (E), exceeding the theoretical value of −1.23V is defined asoverpotential (η) that includes the overpotentials of the anode (η_(a))and cathode (η_(c)) materials as well as other detrimental contributions(η_(o)) attributed to solution and contact resistance and other factors(reference 3).

E=E^(o) _(cell)+η

η=η_(a)+η_(c)+η_(o)

Among known cathode materials, platinum shows the highestelectro-catalytic activity for HER in acidic solutions and the lowestη_(c) that is below −0.1V at the current density of 10 mA/cm². Also theTafel slope, which defines mechanism and efficiency of HER is very lowfor Pt—only about 30 mV/dec in an acidic environment (references 4, 5).

Several types of water electrolyzers are currently in use (reference 6).They include low-temperature electrochemical systems that are based onthe proton exchange membrane technology (PEMEC), which operates inacidic environment, alkaline electrolytic cells (AEC) operating in basicsolutions, and high-temperature water-splitting systems that utilize thedecomposition of vaporized water on solid oxide electrodes (SOEC). Amongthe aforementioned technologies, PEMEC emerges as superior contender,demonstrating high working current density (2 Am⁻²), low operatingtemperature (82° C. vs. 650-1000° C. for SOEC) and, unlike AEC, anability to function under high pressures of H₂ (about 200 bar). One ofthe main weakness of PEMECs, impeding their broad practicalapplications, is high cost and scarcity of platinum (Pt), which is anessential element of their working electrodes.

Layered transition metal dichalcogenides (TMDCs), such as MoS₂, WS₂,MoSe₂, WSe₂ and some others (reference 1), represent a less expensiveand more abundant alternative to Pt. They are built from covalentlybonded metal-chalcogen layers that are held together by weak van derWaals (vdW) forces (FIG. 2). Although thermodynamically stable, bulkTMDCs are rather weak electrical conductors, they show electrocatalyticactivities in HER, which are sufficient for their use in electrochemicalwater splitting systems (references 7, 8, 9).

TMDC-based HER electrocatalysts are usually binary or ternary compoundsthat combine two or three different chemical elements, i.e. transitionmetals and chalcogens (S, Se, Te), in their makeup. In a number of casestwo or more individual (single-phase) TMDCs have been combined incomposite materials (reference 10) that can also contain graphite orgraphene, increasing their electrical conductivity (references 11, 12).

SUMMARY OF THE INVENTION

An embodiment of the present invention provides an electrode materialuseful for electrolysis of water comprising certain transition metalchalcogenides having four or more chemical elements as single phase,solid solution cathode material.

In an illustrative embodiment, the single phase, solid solutionelectrode (e.g. cathode) material is represented by the general formulaof: (M_(a) ¹ M_(b) ² M_(c) ³ . . . M_(n))(X¹ _(A)X² _(B)X³ _(C)),wherein M¹, M², M³ . . . M_(n) each represents a different transitionmetal and wherein the sum of a+b+c . . . n is between 1 and 3, whereinX¹, X², X³ each represents a chalcogen element selected from the groupconsisting of S, Se, and Te and wherein the sum of A+B+C is between 2and 6, and wherein the pristine material preferably does not containplatinum as a principle (major) component.

Illustrative embodiments of such cathode materials include, but are notlimited to, multi-metal and multi-chalcogen materials such as(Mo_(0.5)W_(0.5))SSe, (Mo_(0.4)W_(0.4)Ta_(0.2))SSe and(Mo_(0.475)W_(0.475)Ta_(0.05))SSe as well as multi-metal and singlechalcogen materials such as (W_(0.4)Mo_(0.4)Ta_(0.2))S₂, or(W_(0.475)Mo_(0.475)Ta_(0.2))S₂, Wherein the overpotential and Tafelslopes of the these materials are superior to those of similarly treatedMoS₂ and WSe₂.

Certain embodiments of the invention provide the electrode material inthe form of particles of a size between 0.6 nm and 1 micron (1000 nm)and/or other particulates of a size in one dimension between 0.6 nm and1 micron (1000 nm) in the second and/or third dimensions such asrepresented by nano- or micro-plates, rods, wires, tubes, thin films andsimilar asymmetrical structures.

Other embodiments of the invention provide the electrode material in thebulk form as monolithic sheets, rods, plates, wires, or tubes whoseelectric resistivity is less than 20 Ω-cm. Such bulk forms can be madeby cold and/or hot pressing the particles or by other fabricatingtechniques.

Cathode materials pursuant to certain illustrative embodiments of theinvention are advantageous in that they are stable in acidic aqueousenvironments during water electrolysis and exhibit an overpotentialbelow 0.5V vs. the reversible hydrogen electrode (RHE) at 10 mA/cm²current density during electrolysis of water in acidic environment atroom temperature. The cathode material can have a conductive natureitself or the material can be combined with a conductive material toform a composite cathode material.

Furthermore, other embodiments of the invention provide an electrode(e.g. cathode) comprising a transition metal dichalcogenide substratehaving platinum (Pt) nanosized islands (localized Pt areas) deposited onthe electrode surface. The Pt nanosized islands are less than 50 nm asmeasured along at least one dimension of each localized island. Such acathode electrode behaves electrochemically, in water electrolysis,similar to a platinum metal electrode. A certain method embodiment formssuch an electrode by repeated (consecutive) volt scans performed on thepristine transition metal dichalcogenide cathode substrate in an aqueoussolution in the presence of a platinum counter electrode until thenano-sized islands are deposited on the surface of the electrode cathodesubstrate.

The present invention envisions a water electrolysis cell and methodthat use the electrode material described above as the working cathodeto positively affect the hydrogen evolution reaction.

The present invention and advantages thereof will be described below inmore detail with respect to certain embodiments offered for purposes ofillustration and not limitation in relation to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electrochemical cell forelectrolysis of water.

FIG. 2 shows layered structure of transition metal dichalcogenides(TMDC, MX₂); blue circles represent transition metals (e.g. Mo, W, Taetc.) and yellow circles stand for chalcogens (S, Se or Te).

FIG. 3a shows the LSV data for the studied TMDCs electrodes, wherenumbers correspond to the specific TMDCs: Pt (1), MoS₂ (2), WS₂ (3),(Mo_(0.5)W_(0.5))S₂ (4), (Mo_(0.5)W_(0.5))SSe (5),(Mo_(0.475)W_(0.475)Ta_(0.05))SSe (6), (Mo_(0.475)W_(0.475)Ta_(0.05))S₂(7) and (Mo_(0.4)W_(0.4)Ta_(0.2))S₂ (8) and FIG. 3b shows the same plotslimited to cut-off current density of 10 mAcm⁻².

FIG. 4a shows the Tafel plots of Pt (1), MoS₂ (2), WS₂ (3),(Mo_(0.5)W_(0.5))S₂ (4) and (Mo_(0.5)W_(0.5))SSe and FIG. 4b shows theTafel plots for the Ta containing TMDC materials:(Mo_(0.475)W_(0.475)Ta_(0.05))SSe (6), (Mo_(0.475)W_(0.475)Ta_(0.05))S₂(7) and (Mo_(0.4)W_(0.4)Ta_(0.2))S₂ (8).

FIG. 5 shows resistivity of (Mo_(0.4)Wo_(0.4)Ta_(0.2))S₂ vs.temperature.

FIG. 6 is a scanning electron microscopy (SEM) image showing nanosized(less than 50 nm) platinum islands on the surface of the(Mo_(0.4)W_(0.4)Ta_(0.2))S₂ electrode after 1000 consecutive LSV scans.

FIG. 7a is an X-ray photoelectron spectrum (XPS) of the(Mo_(0.4)W_(0.4)Ta_(0.2))S₂ electrode surface after 1000 consecutive LSVscans and FIGS. 7b-7e show enhanced spectrum areas that correspond tothe signals of main elements forming the electrode. The spectrum of FIG.7a also contains the strong signal of fluorine from the Nation@ binderused for the electrode preparation.

DESCRIPTION OF THE INVENTION

The present invention provides in some embodiments certainmulti-principal element, high-entropy transition metal dichalcogenidematerials (HE-TMDCs) comprising four and more chemical elements andpresent as a single phase, solid solutions for use as electrocatalystsfor the hydrogen evolution reaction (HER) in the electrolysis of water.The TMDCs described below are referred to as high entropy (HE) materialsin that they have unique features of high-entropy materials; namely, acombination of structural irregularities such as random distribution ofdifferent atoms in corresponding metal and chalcogen sub-lattices,lattice defects and vacancies and an enhanced stability such as chemicalresistivity toward acidic environment.

Certain embodiments of the invention involve a cathode material that isrepresented by the general formula of: (M_(a) ¹ M_(b) ² M_(c) ³ . . .M_(n))(X¹ _(A)X² _(B)X³ _(C)), wherein M¹, M², M³ . . . M_(n) eachrepresents a different transition metal and wherein the sum of a+b+c . .. n is between 1 and 3, wherein X¹, X², X³ each represents a chalcogenelement selected from the group consisting of S, Se, and Te and whereinthe sum of A+B+C is between 2 and 6.

Such materials can combine two or more group 4 and/or group 5 transitionmetals, and/or group 6 transition metal, and/or group 9 transitionmetal, and/or group 10 transition metal with one or more chalcogens (S,Se, Te; group 16 chalcogen elements) in their structures. These groupnumbers correspond to those of the Periodic Table of Elements. Moreover,the cathode material does not contain platinum (Pt) as a principal(major) component. The Pt content of the cathode material preferably isless than 10 atomic % to this end.

Certain embodiments provide the cathode material in the form ofparticles of a size between 0.6 nm and 5 micron (5000 nm) and/or otherparticulates of a size in one dimension between 0.6 nm and 5 micron(5000 nm) in the second and/or third dimensions such as represented byplates, rods, wires, tubes, thin films, and similar asymmetricalstructures. Such particle/particulates themselves each can comprise asingle TMDC nano layer or multiple, stacked TMDC layers (FIG. 2), thatare assembled by a self-assembly process or active mixing process toform multiple stacked layered particles/particulates as described inpatent application Ser. No. 15/998,266 filed Jul. 26, 2018, USpublication No. 2019/0039913, the disclosure of which is incorporatedherein by reference to this end.

Bulk forms such as bulk monolithic plates, rods, wires, tubes, etc. forcathode bodies can be made by cold and/or hot pressingpowder/particulates using known powder metallurgy techniques, or byother fabricating methods.

The cathode particles/particulates can be formed by various techniquesinto a working cathode of the water electrolysis cell. Such techniquesinclude, but are not limited to electrophoretic deposition of theparticles/particulates on a substrate (reference 13), drop-casting,(reference 14), or any other electrode preparation technique.

Such particles/particulates can be mixed with any conductive material inparticle, fiber or other particulate form such as graphite, graphene,graphene oxide, reduced graphene oxide, metal, polymer, and/or ceramicsto form a composite material for use as a cathode for the HER that showsoverpotential below 0.5 V vs the reversible hydrogen electrode (RHE) at10 mA/cm² current density during electrolysis of water in acidicenvironment at room temperature. The mixture then can be placed on asubstrate using drop-casting or electrophoretic deposition procedure, orotherwise formed into a working cathode for HER.

Importantly, the working cathode or composite working cathode is stablein an acidic aqueous environment, such as having pH less than 1, and canexhibit an overpotential below 0.5 V vs the reversible hydrogenelectrode (RHE) at 10 mA/cm² current density during electrolysis ofwater in such acidic environment at room temperature. Preferably, theworking cathode can exhibit an overpotential below 0.2 V vs thereversible hydrogen electrode (RHE) at 10 mA/cm² current density duringelectrolysis of water in acidic environment at room temperature.

The present invention also provides in still other embodiments anelectrode (e.g. cathode) comprising a transition metal dichalcogenidesubstrate having platinum (Pt) nanosized particles (localized areas)disposed on the cathode surface as described below.

The following examples are offered to further illustrate and not limitembodiments of the present invention.

EXAMPLES

The following TMDC were used in powder form with particle sizes below 1μm and tested:

-   -   MoS₂    -   WS₂    -   (Mo_(0.5)W_(0.5))S₂    -   (Mo_(0.5)W_(0.5))SSe    -   (Mo_(0.475)W_(0.475)Ta_(0.05))SSe    -   (Mo_(0.4)W_(0.4)Ta_(0.2))SSe    -   (Mo_(0.4)W_(0.4) Ta_(0.2))S₂    -   (Mo_(0.475)W_(0.475)Ta_(0.05))S₂

All materials, except for commercially acquired MoS₂ and WS₂ were madein a powder form by process described in patent application Ser. No.15/998,266 filed Jul. 26, 2018, US. publication No. 2019/0039913, thedisclosure of which is incorporated herein by reference above.

The glassy carbon electrodes, CHI104, were acquired from CH InstrumentsInc. (Austin, Tex., USA) and used for the preparation of the workingelectrodes (i.e. as substrate for the TMDC cathode material). Isopropylalcohol (IPA, ≥99.5%) was purchased from Fisher Scientific (Fair Lawn,N.J., USA) and alumina slurry (particle size=0.05 μm) and the polishingpad (Texmet, diameter=2⅞″, a part of PK—4 Polishing Kit) were obtainedfrom BASi (West Lafayette, Ind., USA). Concentrated sulfuric acid(H₂SO₄≥99.9%, Sigma-Aldrich, St. Louis, Mo., USA) was used to preparethe IM H₂SO₄ electrolyte solutions. Nafion solution (20 wt % in loweraliphatic alcohols and water) was acquired from Sigma-Aldrich (St.Louis, Mo., USA). MoS₂ (99%) and WS₂ (99.8%) were purchased from AlfaAesar (Haverhill Mass., USA). Acetone was purchased from Sigma-Aldrich.

Example 1. Fabrication of TMDC Electrodes by Drop Casting

Prior to the deposition of a TMDC material on the glassy carbonelectrode (CHI104), the electrode was cleaned by polishing its surfacewith the Texmet alumina pad in the presence of the alumina slurry. Thecleaning step was followed by rinsing the electrode with acetone anddeionized water.

The TMDC powder, such as MoS₂, WS₂, (Mo_(0.5)W_(0.5))S₂(Mo_(0.5)W_(0.5))SSe, (Mo_(0.4)W_(0.4) Ta_(0.2))S₂ or(Mo_(0.475)W_(0.475)Ta_(0.05))S₂, was suspended in IPA (5 mg/mL) andsonicated using Branson 3800 ultrasonic cleaner for 2 hours at ambientconditions. An aliquot of about 10 μL of the suspension formed was dropcast onto the glassy carbon electrode and dried for 20 min. at roomtemperature. This procedure was repeated several times. Subsequently,the electrode was dipped into the Nafion® binder solution for about 5seconds and dried in air. Thus prepared working electrode was used forthe electrochemical experiments.

Example 2. Fabrication of TMDC Electrodes by Electrophoretic Deposition

The electrophoretic deposition was carried out in a two electrode,electrochemical cell.

The glassy carbon electrode (CHI104) was used as substrate for the TMDCdeposition and a platinum wire served as the counter electrode.

Specifically, 0.3 g of (Mo_(0.4)W_(0.4)Ta_(0.2))SSe powder was dispersedin 15 mL of IPA and sonicated for 8 hours under ambient conditions.Prior to the electrophoretic deposition, the glassy carbon electrode(CHI104) was cleaned by sonication for 5 min in ethyl alcohol, then for5 min in deionized water. A cyclic voltammogram was recorded from thecleaned electrode and, if no peaks were detected, the electrode wasdeemed clean.

For the electrophoretic deposition, the cleaned glassy carbon electrodeand the platinum wire electrode were placed vertically in anelectrochemical cell with a separation distance of 1 cm between them. Adeposition voltage 10V was applied across the electrochemical cell for15 min using the CH16051E potentiostat from CH Instruments.Subsequently, the working electrode formed was dried at 60° C. for 30min and used for the electrochemical experiments.

Example 3. Electrochemical Experiments

The electrochemical experiments were performed in an electrochemicalcell with a three-electrode configuration using BASi Cell Stand and theEpsilon potentiostat.

The glassy carbon electrode (CHI104) with the deposited TMDC served asworking electrode (WE) for HER and a 2 mm-platinum wire was used ascounter electrode, where OER took place. A silver chloride electrode(Ag/AgCl in 1 M KCl, CH Instruments), calibrated with respect to thereversible hydrogen electrode (RHE), served as the reference electrode,by using high purity H₂ saturated 0.5M H₂SO₄ electrolyte. The linearsweep voltammetry (LSV) scans were performed at the rate of 2 mV/s in H₂saturated 0.5M H₂SO₄ aqueous electrolyte. The acidic aqueous electrolytewas stirred using a magnetic stirrer at a speed of 60 rpm. The LSV datafor all TMDCs were corrected for electrolyte resistance (iR correction).

FIG. 3a shows iR corrected LSV plots for MoS₂, WS₂, (Mo_(0.5)W_(0.5))S₂,(Mo_(0.5)W_(0.5))SSe, (Mo_(0.475)W_(0.475)Ta_(0.05))SSe,(Mo_(0.4)W_(0.4)Ta_(0.2))SSe, (Mo_(0.475)W_(0.475)Ta_(0.05))S₂ and(W_(0.4)Mo_(0.4)Ta_(0.2))S₂. The data represent electrode performancesthat remain constant within about 10 subsequent LSV scans (LSV cycles).For comparison, the performance of a commercial Pt electrode (same ascounter electrode) is also shown.

In FIG. 3b , the 10 mA/cm² current density is selected as the cut-offvalue. The overpotential of the studied TMDC electrodes was defined asthe voltage at which the cut-off current density is obtained—a commonpractice in the field (reference 7). The Tafel slopes of the testedsamples are presented in FIGS. 4a, 4b . The collected electrochemicaldata are also tabulated in Table 1.

TABLE 1 Electrocalalytic activities of the studied TMDC electrodes inhydrogen evolution reaction (HER) Sample Current Density OverpotentialOnset Scan (mA/cm²) (mV) Potential Tafel slope Nr. at 450 mV at 10 A/cm²(mV) (mV/dec⁻¹) MoS₂ 10 0.131 692 539 150 WS₂ 10 2.22 646 375 195.2(Mo_(0.5)W_(0.5))S₂ 500 14.7 430 316 110 (Mo_(0.5)W_(0.5))SSe 500 9.42455 326 120 (Mo_(0.475)W_(0.475)Ta_(0.05))SSe 50 990 260 197 63(Mo_(0.475)W_(0.475)Ta_(0.05))SSe 1000 1430 181 104 55(Mo_(0.4)W_(0.4)Ta_(0.2))SSe* 40 127 232 176 55(Mo_(0.475)W_(0.475)Ta_(0.05))S₂ 50 162 276 190 89(Mo_(0.475)W_(0.475)Ta_(0.05))S₂ 1000 1430 72 48 24(W_(0.4)Mo₀₄Ta_(0.2))S₂ 50 42 239 138 93 (W_(0.4)Mo₀₄Ta_(0.2))S₂ 10001428.6 75 43 32 Pt (wire) 10 1430 62 28 32 *the electrode was fabricatedby electrophoretic deposition

The overpotential and Tafel slopes of the multi-element TMDCs cathodematerials, pursuant to this invention are far superior to the binaryMoS₂ and WS₂ (see Table 1), whereby (Mo_(0.5)W_(0.5))SSe performssimilarly to (Mo_(0.5)W_(0.5))S₂, and all Ta-containing TMDCs are farmore active as HER electrocatalysts than other tested materials. After1000 consecutive LSV scans, the single phase, solid solutions depositedcathode materials represented by (W_(0.4)Mo_(0.4)Ta_(0.2))S₂ and(Mo_(0.475)W_(0.475)Ta_(0.05))S₂ showed electrocatalytic performanceexceeding or close to that of the best electrode materials known todate, including Pt (Table 1).

The (W_(0.4)Mo_(0.4)Ta_(0.2))S₂ and (Mo_(0.475)W_(0.475)Ta_(0.05))S₂materials consisting of four principle elements, exhibit strongimprovements of their performance during LSV scanning and reach thesteady performance state after 500 consecutive scans.

High electrical conductivity of Ta containing materials is at leastpartially responsible for their exceptional performance since it reducesan internal resistivity contributions to their overpotentials.

For example (W_(0.4)Mo_(0.4)Ta_(0.2))S₂ showed metallic behavior, whileconventional TMDCs are indirect band gap semiconductors and have poorelectric conductivity (reference 9). The electrical resistivity of(W_(0.4)Mo_(0.4)Ta_(0.05))S₂ was recorded between 350K and 2K in theabsence of magnetic field using a four-terminal alternating currenttransport setup available in the physical property measurements systemfrom Quantum Design, Inc. The room temperature resistivity of thismaterial is 4.7 mil-cm and it slightly decreases with the temperaturedecreasing into the cryogenic region (FIG. 5). The room temperatureresistivity of (Mo_(0.475)W_(0.475)Ta_(0.05))S₂ is similarly metallic,i.e. 8.5 mω-cm. It is slightly lower due to the lower Ta content in thematerial.

In general, room temperature electrical resistivities below 1 Ω-cm canbe obtained by the TMDC material pursuant to embodiments of the presentinvention.

The present invention also provides in still other embodiments a cathodecomprising a transition metal dichalcogenide substrate having platinum(Pt) nanosized islands or precipitates disposed on the cathode surfaceas described below. As example, FIG. 6 shows SEM image of the platinum(Pt) island precipitates on the (Mo_(0.4)W_(0.4)Ta_(0.2))S₂ electrode,which has been exposed to 1000 consecutive LSV scans in theelectrochemical test cell with the Pt counter electrode. The nanosizedislands comprising Pt are less than 50 nm in size as measured along oneof the dimensions thereof. Referring back to FIG. 6, this cathodeembodiment is advantageous in that such a cathode behaveselectrochemically in aqueous electrolysis essentially very similar tometallic platinum.

The X-ray photoelectron spectroscopic (XPS) examination of this(Mo_(0.4)W_(0.4)Ta_(0.2))S₂ electrode confirmed the presence of Pt onits surface as well as the presence of all other elements including Mo,W, Ta, S and fluorine (F) that is an element of the Nafion conductivebinder coating used for the electrode preparation (FIG. 7).

Deposition of Pt and the performance improvement associated with thepresence of Pt nano-islands on the surface of the TMDC electrode has notbeen observed for the other electrode materials that did not contain Ta.Thus, Ta and, possibly other dopants that improve intrinsic electricalconductivity of TMDCs, can be employed to achieve the observedphenomenon.

Certain illustrative method embodiments of the present invention formsuch a cathode by applying consecutive voltage scans to a TMDC materialthat contains Ta in an aqueous solution in the presence of a platinumcounter electrode until the Pt nanosized islands comprising platinum aredeposited on the material. For example, the cathode can be formed byapplying repeated LSV scans to a pristine conductive Ta-containing TMDCcathode substrate in aqueous solution in the presence of a platinumcounter electrode until the Pt nanosized islands or precipitates form onthe cathode substrate. An exemplary method embodiment forms such acathode electrode using a 0.5M H₂SO₄ aqueous electrolyte and a cathodeelectrode material that contains Ta as described above, a Pt counterelectrode, and a Ag/AgCl reference electrode of the type described aboveand changing the potential (voltage) from 0 V to −0.5V or so. The scansare repeated multiple times until Pt precipitates as nanosized islandson the TMDC cathode electrode.

Alternately, chemical deposition of the nanosized islands comprisingplatinum on a surface of a TMDC material that contains Ta as describedabove can be effected by immersing or otherwise contacting theTa-containing TMDC material in an aqueous solution containing a Ptcompound to effect a chemical reduction reaction (without appliedelectrical current/voltage). The Pt compound can include, but is notlimited to, a water-soluble Pt salt represented by M_(y)PtX_(z), whereinM represents H, Li, Na, K, NH₄ or another appropriate cation and Xrepresents at least one of Cl, Br, I, NO₃ and an organic substituentsuch as C₂O₄, y=1-2, and z=1-6.

Cathode materials pursuant to these illustrative embodiments of theinvention are advantageous in that they are stable in acidic aqueousenvironments, meaning that they do not decay, during water electrolysisand exhibit an overpotential below 0.5 V vs. the reversible hydrogenelectrode (RHE) at 10 mA/cm² current density during electrolysis ofwater in acidic environment at room temperature.

Although the present invention has been described with respect tocertain illustrative embodiments, those skilled in the art willappreciate that changes and modifications can be made therein within thescope of the invention as set forth in the appended claims.

REFERENCES WHICH ARE INCORPORATED HEREIN BY REFERENCE

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We claim:
 1. An electrode material comprising a transition metalchalcogenide having four and more constituent chemical elements andprovided as single phase, solid solution material.
 2. The material ofclaim 1 represented by the general formula of:(M_(a) ¹M_(b) ²M_(c) ³ . . . M_(n))(X¹ _(A)X² _(B)X³ _(C)), wherein M¹,M², M³ . . . M_(n) each represents a different transition metal andwherein the sum of a+b+c . . . n is between 1 and 3, wherein X¹, X², X³each represents a chalcogen element selected from the group consistingof S, Se, and Te and wherein the sum of A+B+C is between 2 and 6, andwherein the material does not contain platinum as a principal (major)component.
 3. The material of claim 2 that combine two or more group 4and/or group 5 transition metals, and/or group 6 transition metal,and/or group 9 transition metal, and/or group 10 transition metal withone or more chalcogens (S, Se, Te; group 16 chalcogen elements) in theirstructures.
 4. The material of claim 1 that contains Ta as a transitionmetal.
 5. The material of claim 4 that comprises at least one of(W_(0.4)Mo_(0.4) Ta_(0.2))S₂, (W_(0.475)Mo_(0.475) Ta_(0.05))S₂,(Mo_(0.475)W_(0.475)Ta_(0.05))SSe and (Mo_(0.4)W_(0.4)Ta_(0.2))SSe. 6.The material of claim 1 that does not contain platinum as a principalcomponent.
 7. The material of claim 1 wherein the content of Pt is lessthan 10 at. % in combination with one or more group 4 and/or group 5transition metals, and/or group 6 transition metal, and/or group 9transition metal, and/or group 10 transition metal with one or morechalcogens, S, Se, Te (group 16 elements) in their structures.
 8. Thematerial of claim 1 that comprises of one or morechalcogen—metal—chalcogen layers.
 9. The material of claim 1 thatcomprises particles of the size between 0.6 nm and 5 micron (5000 nm) inat least one dimension.
 10. The material of claim 1 that comprisesparticles of the size between 0.6 nm and 5 micron (5000 nm) in onedimension and more than 1 micron (5000 nm) in the second and/or thirddimensions wherein the particles include at least one of nano or microplates, rods, wires, tubes, and thin films.
 11. The material of claim 1exhibiting overpotential below 0.5 V vs the reversible hydrogenelectrode (RHE) at 10 mA/cm² current density during electrolysis ofwater in acidic environment at room temperature.
 12. The material ofclaim 1 showing overpotential below 0.1 V vs the reversible hydrogenelectrode (RHE) at 10 mA/cm² current density during electrolysis ofwater in acidic environment at room temperature.
 13. The material ofclaim 1 that is stable in acidic aqueous environment.
 14. The materialof claim 1 that exhibits at least one of metallic electrical resistivityand semi-metallic electrical resistivity.
 15. The material of claim 1that contains multiple transition metals selected from the group Cr, Mo,W, Ta, Nb, V, Hf, Zr, Pd, and Pt.
 16. A composite comprising the singlephase cathode material of claim 1 and a conductive material.
 17. Thecomposite of claim 16 wherein the conductive material comprises one ormore of graphite, graphene, graphene oxide, reduced graphene oxide,metal, polymer, and ceramic in particulate form that show overpotentialbelow 0.5 V vs the reversible hydrogen electrode (RHE) at 10 mA/cm²current density during electrolysis of water in acidic environment atroom temperature.
 18. The composite of claim 16 that is stable in acidicaqueous environment.
 19. In a water electrolysis cell, a cathodecomprising the cathode material of claim 1 for the hydrogen evolutionreaction.
 20. A cathode comprising a transition metal dichalcogenidesubstrate having nanosized islands comprising platinum disposed on thecathode.
 21. The cathode of claim 20 wherein the nanosized islands eachare less than 50 nm in size as measured along one dimension.
 22. Thecathode of claim 20 wherein the transition metal dichalcogenide includesTa as a transition metal.
 23. The cathode of claim 20 that comprises atleast one of (W_(0.4)Mo_(0.4) Ta_(0.2))S₂, (W_(0.475)Mo_(0.475)Ta_(0.05))S₂, (Mo_(0.475)W_(0.475)Ta_(0.05))SSe and(Mo_(0.4)W_(0.4)Ta_(0.2))SSe.
 24. A method of forming a cathodeelectrode, comprising applying consecutive voltage scans to a TMDCmaterial in an aqueous solution in the presence of a platinum counterelectrode until the Pt nanosized islands comprising platinum aredeposited on the material.
 25. The method of claim 24 wherein the TMDCmaterial includes Ta.
 26. A method of forming a cathode electrode,comprising chemical depositing nanosized islands comprising platinum ona surface of the TMDC material of claim 1 in aqueous solution using Ptcompound in solution.
 27. A method of claim 26, where the Pt compoundcomprises a water-soluble Pt salt represented by M_(y)PtX_(z), wherein Mrepresents H, Li, Na, K, NH₄ or another appropriate cation and Xrepresents at least one of Cl, Br, I, NO₃ and an organic substituent,y=1-2, and z=1-6.